Cutting insert

ABSTRACT

A cutting insert for material-removing machining is formed with at least one generally straight cutting edge and a central flat plateau spaced backward from and at a predetermined height above the cutting edge. A plurality of elongated ridge-like chip-forming elements each extend along a respective longitudinal axis forming a respective acute angle to a respective perpendicular to the cutting edge from a leading end spaced backward from the cutting edge to a trailing end at the plateau and are each formed of a row of discrete bumps defining a contour line having adjacent the rear end a rear portion of a maximum height at least equal to that of the plateau, forward therefrom a front portion of a minimum height, and a rising intermediate portion having a length equal to at least half a length of the respective element. The cutting insert further is formed with a land extending along the cutting edge between the front ends and the cutting edge and a groove extending parallel to the cutting edge between the land and the plateau, the chip-forming elements being mainly in the groove.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of PCT applicationPCT/DE93/01108 filed 18 Nov. 1993 with a claim to the priority of Germanapplication P 42 39 236.5 itself filed 21 Nov. 1992.

FIELD OF THE INVENTION

The invention relates to a polygonal cutting insert with on the cuttingsurface raised chip-forming elements that are arranged in a row next toone another at a spacing from the cutting edge and which are eachelongated with a respective longitudinal axis which forms an acute anglewith a perpendicular to the cutting edge.

BACKGROUND OF THE INVENTION

Such a cutting insert is known from U.S. Pat. No. 4,969,779.

A cutting tool is known from German 2,231,631 having a chip-breakinggroove in which at least one frustoconical projection is provided in acutting corner.

It has also already been suggested to arrange a plurality ofpart-spherical chip-forming elements next to one another in achip-breaking groove.

The known cutting inserts are nonetheless, due to the arrangement of thechip-forming elements and the geometry of the chip groove, only usablein a limited range of different cutting conditions, as with various cutdepths and feed rates. In order to improve the usability of the cuttinginserts and to better shape and remove the chip German 3,148,535suggests forming the chip-forming elements generally frustoconicallywith generally triangular base surfaces, each having a base line alignedparallel with the adjacent cutting edge or on an arcuate line.

According to German 2,819,824 a cutting insert is suggested having inits center a boss (plateau) which is formed as a polygon and whose basicshape corresponds to that of the cutting insert. In particular thecorner of the rectangularly shaped boss is aligned to the center of thecutter but is otherwise formed annularly. Such cutting inserts are alsoknown where the chip-forming boss extends right up to the cutting cornerregion. The disadvantage of such embodiments is that during cutting highpressure is produced which can lead to premature wear of the cuttinginsert. In addition the achieved cut quality is unsatisfactory.

In order to ensure uniformly good chip formation at widely differentfeed rates, cut depths, and cutting speeds it has been suggested inEuropean 0,278,083 to provide a cutting angle of more than 30° on theland adjacent the cutting edge, the cutting angle decreasing away fromthe land, and chip ribs projecting outside the cutting surface areprovided in the cutting surface region extending in the chip-traveldirection and between the cutting-corner regions of the cutting insert,between which chip ribs further bumps with run-on ramps are provided.The corner region has three adjacent spur bosses of which the two outerones have their chip longitudinal axes parallel to the adjacent chipribs.

Run-on ramps are also provided in the (plunge) cutting insert accordingto U.S. Pat. No. 4,992,008, these ramps being directly at the cuttingcorners and extending as a wide roof surface in the direction away fromthe cutting edge of an elongated chip-forming element with a trapezoidalshape.

The cutting inserts described in European 0,168,555 and 0,222,317 have acentral plateau that is raised relative to the cutting corners andcutting edges and from which part-spherical projections extend towardthe cutting edge and extend if necessary into a chip-forming grooveprovided there.

European 0,414,241 describes cutting inserts with pits which interruptthe cutting edge and thus form groove-like cutting-edge formations. Thisarrangement nonetheless has a wearprone cutting edge that is likely tochip.

Part spherical chip-forming elements do indeed lead during chipformation to a desired chip formation but the chip is neither laterallysufficiently guided in the chip-travel direction so that lateralbreaking of the chip can lead to over deformation of the chip and noactual chip breaking or insufficient chip breaking. Longitudinallyelongated chip-forming element guide the chips better in the traveldirection but if the contact surface on the rib is to small the chipeasily can break off to the side. The mentioned chip run-on ramps aredifficult to make and subject to wear.

OBJECTS OF THE INVENTION

Starting from this state of the art the object of the present inventionis to improve the above-discussed cutting inserts with respect to theformation of the chip-forming elements so that with a relatively largestarting curve of the chip, in particular when the chip former is usedwith a small feed rate, there is a particularly large bending of thechip. A further object is that the cutting insert should be usable inthe widest possible feed region so that with greater advance anexcessively small starting curvature or too tight a rolling of the chipis avoided.

SUMMARY OF THE INVENTION

A cutting insert for material-removing machining is formed with at leastone generally straight cutting edge and a central flat plateau spacedbackward from and at a predetermined height above the cutting edge. Aplurality of elongated ridge-like chip-forming elements each extendedalong a respective longitudinal axis forming a respective acute angle toa respective perpendicular to the cutting edge from a leading end spacedbackward from the cutting edge to a trailing end at the plateau and areeach formed of a row of discrete bumps defining a contour line havingadjacent the rear end a rear portion of a maximum height at least equalto that of the plateau, forward therefrom a front portion of a minimumheight normally therebetween a rising intermediate portion having alength equal to at least half a length of the respective element. Thecutting insert further is formed with a land extending along the cuttingedge between the front ends and the cutting edge and a groove extendingparallel to the cutting edge between the land and the plateau, thechip-forming elements being mainly in the groove.

The invention is based on the following considerations:

Chip-breaking is based on deforming the cross section of the passingchip until it breaks. Chip-breaking can be influenced both by the chipcross section and by the starting bend arc which is imparted to the chipby the chip-forming element of the cutting insert. It is indeed knownthat chip breaking is enhanced by increasing the thickness of the chip,however the chip thickness is largely determined by the material beingmachined and other machining conditions.

Producing the smallest possible starting bend radius to improve chipbreaking has generally the disadvantage of greater cutting forces andthe associated increased load on the cutting insert. This is added todynamic forces that can lead to impermissible vibrations in the cuttingprocess both with respect to frequency and amplitude. This isparticularly critical under unstable machining conditions, as forexample for thin work-pieces, when plunging, with inside machining, andwith drilling. In extreme cases the chips can jam up in the chip formerand break the tool.

On the other hand the starting curvature radius can also not be selectedat any large size since the chip must be bent further in order to break.Accordingly there are generally longer and "opener" chips, chip curls,and chip pieces. In this manner the danger grows that the chip isinsufficiently deformed by the chip former and, as a result of theeffect of the forces and moments which are associated with thechip-breaking process as well as because of its size, it is in theposition to deviate from the obstruction formed by the chip-formingelement. This can lead to undesired helical chips. The longitudinal ribsaccording to the invention deform the chip cross section in grooves nearthe blade where the chip is easily plastically deformed. The spacing ofthe ribs from the cutting edge is maintained at a minimum to reduce theeffects of friction so that the danger of breakage at the cutting edgeis kept small. The longitudinal ribs provide due to their formation, inparticular their length, better chip guiding which counters lateraldeflection of the chip. As a result the chip can be bent to a largeradius for breaking. The high point in the region spaced from thecutting edge or the inclination of the raised longitudinally extendingchip-forming element up to the height of the central cutting-surfaceplateau ensure that even with greatly bent chips lateral guiding is notreduced or lost. In particular the contact of the passing chip and thusthe friction on the chip-running surface is reduced to a minimum so thatgreater service life for the cutting inserts is achieved. The formationof the cutting surface by producing intermediate spaces between the ribsensures exceptional accessibility for coolant which also helps toincrease the life (service life) of the cutting insert. Finally the ribshave a stabilizing effect on the cutting edge.

Further features of the invention serve to produce an exponentialposition of the rib high points along with a convex curvature of the ribprofile to the summit point so that additional space is created in orderto over stretch the chip and bend it around. The described curvaturesare increased by the groove formation when the chip as result ofengagement against an obstruction is pressed against the cutting insert.

With higher feed rates the ribs minimize the danger of the chips jammingup, and of vibrations since as a result of the thinness of the rib thechip can be formed are "rolled open." Particular advantages arepossessed by the cutting-insert geometry which is used by drills intowhich the cutting insert is set since according to overlap and drilldiameter the chip or chips produced from the two cutters are generallycentrally guided or laterally guided and deflected and rolled upconically.

In particular so-called short-hole drills use W-type cutting insertswhich are of triangular shape with the cutting edges having aroof-shaped projection forming an obtuse angle.

In order to use such cutting inserts both for right- and left-handedturning and inside and outside cutters the chip-forming elements areformed and arranged symmetrically with respect to a longitudinal axisthat extends as a bisector through a cutting corner and through thecenter of the opposite cutting edge, namely at its obtuse-angle corner.

In particular with indexable cutting plates with a bore seat and aboveall with positive indexable cutting plates with conical bit holes thereis limited space available on the cutting surface. In order toaccommodate these relationships the high point of the rib crest remotefrom the cutting edge or its height above the cutting-surface plateaudecreases uniformly or in steps from the sharp (cutting) corners to theobtuse corners or the center of the cutting edge. A correspondinguniform or step-wise decrease is also provided with respect to thedistances, that is the height differences, between the rib crest and thebase surface in the same described region. It is also possible toprovide for uniformly or step-wise decreasing spaces of the rib pointsto the cutting edge from the (acute) cutting corners to the obtuse-anglecorners. The goal of the described features is to achieve sufficientstability and protection against a dip collar at the narrowest placewith uniform chip bending over the width of the chip.

Alternatively it is also possible to form the central rib between acutting corner and the middle of the cutting edge somewhat higher thanthe adjacent ribs in order to there achieve a greater deformation of thechip cross section.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are illustrated in the drawings. There isshown in:

FIG. 1 a top view of a generally rectangular cutting plate with twocutting edges and four respective chip-forming elements per cuttingedge;

FIG. 2 a triangular cutting plate with six chip-forming elements percutting edge;

FIGS. 3a to 3h longitudinal sections along a chip-forming element;

FIGS. 4a to 4c cross sections taken along lines IVa--IVa, IVb--IVb andIVc--IVc of FIG. 3h;

FIG. 5a a longitudinal section through a chip-forming element,

FIG. 5b is an end view showing the outlines of the three sections shownat Vb1--Vb1, Vb2--Vb2, and Vb3--Vb3 of FIG. 5a;

FIGS. 5c through 5p are views analogous to FIG. 5b but showing differentconfigurations;

FIG. 6a a longitudinal section through another embodiment of thechip-forming elements;

FIG. 6b a top view of the chip-forming elements according to FIG. 6a and

FIGS. 7a and 7b similar views showing chip travel with a cutting insertaccording to the invention (FIG. 7b) and with a cutting insert accordingthe prior art (FIG. 7a);

FIG. 8a a top view of a further embodiment of a cutting insert;

FIG. 8b a schematic top view of the cutting insert according to FIG. 8afitted to a drill bit during cutting in a bore hole;

FIGS. 9a to 9d various cutting inserts in top view;

FIGS. 10a through 10c sections along sections lines Xa--Xa, Xb--Xb, andXc--Xc of FIG. 9a through 9d and

FIG. 11a through 11c corresponding sections through a further embodimentand

FIG. 12 a top view of an exclusively left- or right-cutting plate.

SPECIFICATION DESCRIPTION

The cutting insert 10 shown in FIG. 1 has a generally rectangular shapewith four cutting corners 11 which define the cutting surface 12 as wellas the cutting edges 13 and the longitudinal side edges 14. As known inthe art the cutting insert can have a mounting hole (not shown) Each ofthe two opposite cutting edges 13 has a central setback 15 as well assymmetrically to both sides thereof two rounded projecting regions 16aand 16b. Inward from each corner 11 the cutting insert 10 has extensions17 which are generally parallel to the longitudinal axis L. Theseextensions 17 serve during use when the cutting insert is set in a drillbit as auxiliary cutting edges. The auxiliary cutting edges each have alength which should correspond to the feed per revolution of the bit. Itis somewhat in the range up to 4/10 mm. To bridge between the auxiliarycutting edges 17 and the long side surfaces 14 there is a bevel whichruns at an angle up to 25°. The cutting insert has raised chip-formingelements formed as elongated ribs whose longitudinal axes are directedso that they run in the direction of chip travel. In the case of arectangular cutting insert according to FIG. 1 this is concrete in thatthe angle of the longitudinal axes of the chip-forming elements 18 aresmaller inward on the setback 15.

FIG. 2 shows a generally triangular cutting insert 19 which has threecutting edges 20. The rib-shaped chip-forming elements whose axes 21alone are shown also run generally in the chip-travel direction so thatthe angle of the longitudinal axes 21 with a central perpendicular of acutting edge are smaller toward the center of the cutting edge. Thecutting inserts shown in FIGS. 1 and 2 are only by way of example,basically all known polygonal shapes as well as indexable plates withpart-circular blades can be equipped with the illustrated rib-shapedchip-forming elements. The ribs 18 can have a uniform width along theirentire longitudinal axis or can widen toward the region remote from thecutting edge as shown in FIG. 1.

The different shape possibilities of the longitudinal profile are shownin FIG. 3.

As can be seen in FIG. 3a behind the cutting edge 13 or 20 a land 22 isfirst defined, here at an angle of 0°.

After the land is a chip-forming groove 24 with a descending flank part241, a flatly descending part 242, and a rising flank 243 up to aplateau 23 in the middle of the cutting surface. The entire chip-forminggroove 24 extends past the chip-forming element 18 that runs at its endtoward the cutting edge at the height of the land 22 and in the middlehas a descending region 181, then a slightly rising region 182, andfinally a convexly curved region 183 with this convex curvature reachinga high point that is above the plane defined by the plateau 23. Thechip-forming element runs into the plateau "behind" the chip-forminggroove 24. In contrast to the embodiment according to FIG. 3a theembodiment according to FIG. 3b follows the land 22 with a chip-forminggroove 25 behind which is a further groove-shaped recess 26 that forms aslight recess whose biggest region forms a climbing flank to a raisedplateau 27 that projects clearly above the cutting edge or the adjacentland 22. The chip-forming element runs past the descending part of thefirst chip-forming recess 25, that is into the land 22. Thischip-forming element has a slightly wavy rib-comb shape with a first lowpoint 184 as well as a slightly convex bump 185 in the rising region toa high point 183. The chip-forming rib ends in the region of the plateau27 somewhat behind the edge of the climbing flank 261.

The chip-forming recess of the embodiment according to FIG. 3ccorresponds generally to the chip-forming recess of FIG. 3a so referenceto this embodiment is made. The same applies to the land 22 and theplateau 27 (see description of FIG. 3b). The chip-forming element 18runs generally centrally into the land 22 at the cutting edge 13 or 20and continues in the direction of its longitudinal axis beyond a highpoint 186 as a concave region whose central zone 187 runs parallel tothe plateau surface 27 or the land 22 but below same. Following thishorizontally oriented zone 187 the chip-forming element 18 has aclimbing flank 188 that rises to a height corresponding to the plane ofthe plateau 27. Outward from this point the chip-forming element goesinto a horizontal part 189 and ends at the edge of the plateau 27 at aclimbing flank 243 of the recess 24. The chip-forming element accordingto FIG. 3d has relative to the rib comb according to FIG. 3c a morehighly defined high point 186 as well as a central region 187 which islightly concave with a large radius of curvature. The high point 183' isvirtually pointed so that the rising and falling flanks of this highpoint are nearly straight seen longitudinally.

In the embodiment of FIG. 3e the land 22 merges into a descendingconvexly curved chip-forming part 28 with a recessed middle part 29 inwhich a bore 30 is also shown. The chip-forming element 18 merges via aflatly extending high point 186 into a convex region 187 as well as ahigh point 183 that itself lies below the plane defined by the land 22.The chip-forming element 18 ends in the region of the recessed centralpart 29.

In the embodiment according to FIG. 3f the cutting edge 13 or 20 has apositive land or is followed by a descending region 31 of a chip-forminggroove. The chip-forming rib starting at a spacing from the cutting edge13 or 20 has a crest which to start with drops slightly, runs through avalley region, and ends in a steep climbing flank to a saddle-shapedplateau 32 before the descending flank drops to the central plateau 27of the cutting insert.

The cutting insert according to FIG. 3g has a negative land with arising angle at which the chip-forming rib rises to a first high point186. The chip-forming rib drops steadily from this high point 186 withits crest alternately convex and concave to a high point 183. Thefalling flank from the high point 183 to the central plateau 27 isstraight. The embodiment of FIG. 3h has a chip rib which ends in thefalling part of the chip-forming recess 24 and has differentcross-sectional shapes running to the regions remote from the cuttingedge as visible in the sections taken along lines A, B, and C of FIGS.4a to 4c. A further feature of the chip-forming rib 18 of thechip-forming rib crest or the upper part or saddle is seen in FIG. 5a.Comparing the spacing of the crest of the chip-forming rib to thechip-forming region at various spacings from the cutting edge 13 or 20it can be determined that these spacings grow continuously away from thecutting edge.

FIGS. 5b to 5p show various sectional views along lines I--I, II--II,and III--III generally parallel to the cutting edge. In order to makethe changes in section between the sections clearer the respective basesof the sections are set straight and one over the other although forexample the chip-forming groove in section II--II is deeper than insection I--I.

FIGS. 5b to 5p show that all sections have the same shape and it canalso be seen how the cross-sectional area grows with respect to heightand width away from the cutting edge. Specifically the chip-forming ribcan have a trapezoidal cross section (FIGS. 5b to 5d or 5h). The crosssections are different in the base width, the base being the surface inthe region of the cutting surface as well as the width of the flatlyformed rib crest. Similarly the flanks falling from both sides of thecrest can be different. As visible from FIG. 5e the crest can be changedfrom an upper flat surface at the high point 183 to a round shape or asshown in FIGS. 5f and 5g as well as 5k in a convex shape. In contrastaccording to FIG. 5i the chip-forming crest can be flat in the rear endremote from the cutting edge and rounded in the front regions. FIG. 5mshows that different angles are possible between the individual sectionplanes. The chip-forming rib 18 according to FIGS. 5n and 5o have anupper point 32 from which the lateral flanks are either wholly flat(FIG. 5o) or convex to start with and then concave to the surface 33which if necessary can be formed as a groove. As visible from FIG. 5pthe side flanks 33 can have a convexity which is tangential at the base.

For all shapes according to FIGS. 5b to 5p of course such contour shapescan be employed as shown in FIGS. 3a to 3h and described above.

According to a further embodiment of the invention the ribs can beformed of a plurality of succeeding and touching or spaced bumps 34having contour line 35 or 36. The same is possible for theabove-described ribs (see FIGS. 6a and 6b).

The operation of the cutting insert according to the invention isvisible from FIG. 7 which in the representation of FIG. 7a shows thepassing chip with a cutting insert known from the prior art and in FIG.7b shows a cutting insert according to the invention. On rotation of theworkpiece 37 a conventional cutting insert 38 produces a long chip 39which in spite of the chip-forming groove and chip-guiding step leadingto a plateau of the cutting insert does not break so that long coiledchips are produced. The chip 40 produced by the cutting insert accordingto the invention works otherwise because of its excess deformationand/or its better lateral guiding so that after engagement with theworkpiece 37 a break (see arrow 41) occurs for the desired short chipformation.

The cutting insert shown in FIG. 8a has a triangular shape with three(pointed) cutting corners 51 and three blunt cutting edges 52 that forman angle of about 160 ° and that are located centrally between twocutting corners 51. If a longitudinal axis 53 is drawn through a pointedcutting corner 51 and the opposite obtuse cutting corner 52, thechip-forming elements 18 are arranged mirror-symmetrical to therespective longitudinal axis 53. This includes as shown in a FIG. 8asuch embodiments where a longitudinal axis 53 is also the longitudinalaxis of a chip-forming element 18 or lies on the longitudinal axis 53.

This way the cutting insert can be used in right- and left-hand turningtools as inside and outside cutters. The orientation of the chip-formingelement satisfies the requirements that the inner ribs are tippedrelative to the central axis by an angle from -5° to 13° and the outerribs by an angle of -5° to 200°.

FIG. 8b shows cutting inserts according to FIG. 8a in use in drill.Within a bore wall the outer cutting insert cuts with a cutting widthb_(a) and the inner cutting insert with a cutting width b_(i). The boreaxis is shown at 55.

FIGS. 9a to 9d show different cutting inserts 56 to 59 with a mountinghole 60.

As visible individually with reference to the sections A--A, B--B, andC--C in FIGS. 10 and 11, different embodiments are usable. In particularwith rhombic or triangular indexable cutting plates, e.g. the standardD, K, V, W shapes, the high point remote from the cutting edge of therib crest or its part 189 level with the cutting surface plateau lies atdifferent spacings from the cutting edge seen from the sharp corner tothe obtuse corner (section C--C to section A--A), that is the heights h₃increase continuously above h₂ and h₁. In this manner the cuttinginserts have sufficient stability as well as protection against sinkinginto the bore 560 at the tightest part. The chip is bent smoothly overthe cutting width.

The spacings (height differentials 1₁ to 1₃) between the crest of therib arc and the base surface 63 can also decrease smoothly or in stepsfrom the blunt or pointed corners to the middle of the cutting edge.Finally the spacing of the rib high points can decrease smoothly or insteps from the cutting edge to the sharp to the blunt corners.

A further possibility of varying the geometry of the ribs and/or of thebase surface on a cutting insert is shown in FIG. 11 where the centralrib acting on the span is particularly prominent according to sectionB--B, that is has a big height H₂. This height becomes greater than h₃with h₁ =0.

FIG. 12 shows by means of the illustrated rib longitudinal axes 62 acutting insert for left- or right-handed machining.

We claim:
 1. A cutting insert formed with:at least one generallystraight cutting edge; a cutting surface extending to the edge; acentral flat plateau spaced backward from and at a predetermined heightrelative to the cutting edge and to the cutting surface; and a pluralityof continuous, elongated, ridge-like chip-forming ribseach surrounded byand projecting wholly above the cutting surface, each extendingcontinuously along a respective longitudinal axis forming a respectiveacute angle to a respective perpendicular to the cutting edge from aleading end spaced backward from the cutting edge to a trailing end atthe plateau, each having a continuous crest defining a contour linehaving adjacent the respective rear end a rear portion of a maximumheight above the cutting surface at least equal to that of the plateau,forward therefrom a front portion of a minimum height above the cuttingsurface, and a rising intermediate portion above the cutting surface andhaving a length equal to at least half a length of the respective rib.2. The cutting insert defined in claim 1 wherein the insert is furtherformed witha land extending along the cutting edge between the leadingends and the cutting edge; and a groove extending parallel to thecutting edge between the land and the plateau, the chip-forming ribsbeing mainly in the groove.
 3. The cutting insert defined in claim 1wherein each rib has a rear portion having the maximum height and of alength equal to at most one-third of the length of the respective rib.4. The cutting insert defined in claim 2 wherein each contour linedefines a supplemental front high portion between the respective frontportion and the respective intermediate portion and having a heightgreater than that of the cutting edge.
 5. The cutting insert defined inclaim 2 wherein the ribs have side surfaces extending at an angle of atmost 80° to a plane of the insert.
 6. The cutting insert defined inclaim 2 wherein the length of the ribs is between 0.7 mm and 5 mm. 7.The cutting insert defined in claim 2 wherein each rib has apredetermined width between its ends, the length of each rib being 1.2to 4 times its width.
 8. The cutting insert defined in claim 7 whereinthe width of each rib decreases toward the cutting edge.
 9. The cuttinginsert defined in claim 8 wherein each width is at a maximum within adistance equal to one third of the respective length from the respectiverear end.
 10. The cutting insert defined in claim 2 wherein the ribs areof generally constant width.
 11. The cutting insert defined in claim 2wherein the rear portion is between 0.05 mm and 1.5 mm high.
 12. Thecutting insert defined in claim 2 wherein each rib has a predeterminedwidth and is spaced from adjacent ribs by a distance equal to between1.5 and 10 times the respective width.
 13. The cutting insert defined inclaim 2 wherein the insert is polygonal having a plurality of sides eachconstituting a respective cutting edge and each formed with a respectiveplurality of the ribs.
 14. The cutting insert defined in claim 2 whereinthe insert is rectangular and has two opposite sides each formed with arespective such cutting edge each having a respective plurality of theribs.
 15. The cutting insert defined in claim 14 wherein the insert hastwo flanking sides bridging the cutting edges and each forming with therespective cutting edges a pair of laterally projecting cornerextensions.
 16. The cutting insert defined in claim 14 wherein eachcutting edge is symmetrical to a longitudinal axis and has four suchribs symmetrically flanking the axis with two inner ribs and two outerribs, the former being spaced from the axis by a distance equal to 5% to20% of the length of the respective cutting edge and the former by adistance equal to 35% to 45% of the respective cutting-edge length. 17.The cutting insert defined in claim 2 wherein the leading ends arespaced at between 0.05 mm and 0.7 mm from the cutting edge.